The present invention relates generally to a method for producing titanium alloy brazing strips and the resulting brazing strips or foils. More particularly, the invention relates to: a titanium based multi-layer alloy strip or foil made up of discrete layers of titanium and an additional metal or metals, such as nickel or nickel alloys and/or copper or copper alloys, for example; a titanium based multi-layer alloy strip or foil made up of discrete layers of titanium, zirconium and an additional metal or metals, such as nickel or nickel alloys and/or copper or copper alloys, for example; and a method for using a cold-rolling process to generate a titanium based multi-layer alloy strip or foil made up of discrete layers of titanium, zirconium and/or additional metal or metals, such as nickel and/or copper, for example.
Brazing alloys based on titanium (Ti) are useful for brazing components that consist of titanium, nickel (Ni) and/or iron/steel (Fe) based elements or alloys, among other uses. Thin gauge brazing strips or foils have proven useful for filling braze joints, and/or for providing suitable substrate materials to form self-brazing bonds and/or for vacuum brazing. The lower melting points of common Ti-based brazing alloys cause a beneficial minimum effect on the microstructures and mechanical properties of the brazed components. Furthermore, Ti-based brazing alloys tend to provide corrosion resistance that is superior to conventional copper (Cu) or silver (Ag) based brazing alloys. A roll bonding process is useful for allowing desirable brazing alloys to be produced in continuous coil form in thin gauge. The availability of these Ti-based brazing alloys in thin foil gauge and in continuous coil lengths has been difficult to achieve, as these alloys tend to be brittle and render the conventional cold working process difficult to utilize. Cold rolling titanium typically results in a brittle metal. Thus, a means for obtaining Ti based brazing alloys and/or compounds and/or laminates in a foil form using conventional cold working techniques would be useful.
Beta Ti alloys with a body-centered-cubic crystalline structure are stabilized by the addition of beta stabilizers such as molybdenum (Mo), Zr, Ni, or Nb to Ti. These alloys show superior formability than the conventional alpha or alpha-beta Ti alloys. Beta Ti alloys can be cold rolled to thin gauge and formed into complex parts such as the fins in a honeycomb structure. Brazing is the favored joining method to provide structure integrity and ease of manufacturing. However, Ti alloy, with its highly reactive nature, readily forms stable scales that prevent conventional brazing alloys from wetting the surface. Ti-15Ni-15Cu type brazing alloy was developed as the brazing alloy of choice for Ti alloys, such as in British patent no 1141247, for example, incorporated herein by reference.
Ti alloys suffer the beta transus phenomenon that results in an undesirable microstructure after a brazing cycle. The beta transus of Ti alloys refers to the temperature at which Ti undergoes phase transformation (alpha to beta or vice versa) and results in crystal structure changes. In the beta type Ti alloys, the alpha phase tends to precipitate at the beta grain boundary at a temperature above the beta transus, which causes embrittlement that is detrimental to the ductility and fatigue resistance of the materials. The brazing temperature has to be kept as low as possible and holding time has to be minimized as well to avoid the aforementioned embrittlement. This disclosure describes brazing alloys of Ti—Cu—Ni and also Ti—Cu—Ni with zirconium (Zr) addition to reduce the melting (brazing) temperatures significantly below that of the Ti-15Cu-15Ni brazing alloys.
In the arrangement of the components of a multi-layered brazing alloy, it would often be useful to have the Ti or Zr layer in the middle. The advantages of having the Ti or Zr layer in the middle would be the resulting uniform relative thickness of the Cu/Ni, Ni/Cu/Ni or Ni/Zr/Cu layers to the middle Ti layer or the Ni/Ti/Cu layers to the middle Zr layer as well as the homogenous deformation of the composite during the cold reduction. These advantages are often important to provide uniform chemistry and thin finish thickness for brazing shim application.
This is in contrast to the Ti/Cu—Ni/Ti arrangement cited in the U.S. Pat. No. 3,652,237 (incorporated herein by reference). In that patent, the Ti layers are on the outside of the relatively soft Ni—Cu layer. The Ti layers are hermetically welded to form an envelope to sandwich the Ni—Cu layer. A few drawbacks can result from this arrangement. The exposed, reactive Ti layers may not permit the heat treating of the composites, because it is conducted in air or hydrogen or nitrogen as Ti reacts and forms easily Ti oxide, hydride and nitride with the respective heat treating atmospheres. This leaves the heat treating typically only feasible in a vacuum, which is typically not a process that can be performed in a continuous, strip-annealing manner. The hard Ti layers on top of the soft middle Ni—Cu layer can also introduce non-uniform deformation of the softer middle layer. The non-uniform deformation of a center soft layer can affect the local alloy chemistry by deviating from the intended composition required by the brazing. This type of localized, non-uniform deformation of the center layer can also post a limit on the minimum thickness that strip can reach before the local asperity leads to a fracture of the materials.
It would be useful to have a strongly adherent, multi-layered composite produced by a roll bonding process avoiding some or all of the above problems. The roll bonding process has a few advantages over the other approaches such as plating or hot bonding. It would be advantageous to utilize a roll bonding process to provide a large reduction in thickness (such as greater than 60%, for example) during the bonding of the components in the brazing alloy. The large reduction, by breaking up the surface scale, would allow a true metallurgical bond to form between the dissimilar materials. Because the temperature of roll bonding process can be advantageously low, there is little concern of possible alloy diffusion or scale formation, especially if heat treating steps can be avoided. The bond integrity could allow the composite to be processed to the desirable thickness, preferably without any intermediate heat treating to soften the materials. However, using a Cu—Ni alloy, especially in a near-equal weight percent condition, typically results in significant hardening if utilized in a cold rolling process, and thus requires intermediate heat treating steps to get to a sufficiently thin gage. A process that does not require the heat treating step could provide savings in time and money.
Furthermore, intermediate heat treating is often not desirable because brittle compounds between the constituent layers might form and render any further cold reduction difficult or even impossible. One advantage of a roll bonding process is to allow the strips to be bonded at heavy thickness, followed by the conventional cold reduction process, and thus providing a higher throughput than another process such as plating. Furthermore, the roll bonding process allows the adjustment of the relative thickness of individual constituents in order to tailor the chemical composition of brazing alloys. In addition, eliminating any heat treating processes simplifies the manufacturing process and reduces energy costs. Consequently, providing a means for using a cold rolling process without heat treating to generate the desired Ti alloy thin sheets and foils for brazing would be beneficial.
The addition of Zr to the Ti—Cu—Ni brazing alloy would allow melting (brazing) to occur at lower temperatures than does a Ti—Cu—Ni brazing alloy without Zr. The lowered brazing temperature results in reduced undesirable microstructure changes in the brazed parts after brazing. Reduced microstructure changes cause less embrittlement of the brazed parts. Because embrittlement is detrimental to the ductility and fatigue resistance of the brazed parts, the use of a Ti—Cu—Ni brazing alloy with added Zr, and the concomitant lowered brazing temperature, would result in brazed materials with better mechanical properties than would otherwise be obtained using a Ti—Cu—Ni brazing alloy without added Zr. The addition of Zr to the Ti—Cu—Ni system has only been reported by the mixed powders (see EU patent 0456481A2, incorporated herein by reference) and rapid solidification produced thin foil (see U.S. Pat. No. 6,475,637, incorporated herein by reference). The rapid solidified foil is limited in the width and quantity, while the powders suffer the risk of the contamination that prevents uniform wetting of the base materials. Therefore, it would be beneficial to provide a means for using a cold rolling process to generate the desired brazing foils and sheets made of an alloy of Ti, Zr, and other metals.